Force commutation circuits

ABSTRACT

A commutation circuit is disclosed for commutating power-control switching devices, such as thyristors or silicon-controlled rectifiers, utilized for supplying a load from a DC source, wherein a commutating capacitor is operatively connected across the power control switching device to be commutated by selectively turning on commutation-controlled switching devices, which may also comprise thyristors or silicon-controlled rectifiers. Also unidirectional devices are employed to provide a circuit path for any reactive energy trapped in the load circuit during the commutation operation.

United States Patent John: P ourgh,Pa. 21] AppLNo. 412

[72] Inventor [22] Filed iar. 11,1970 [45] Patented Oct. 19, 1971 l 7 3Assigne Westinghouse Electric Corporation Pittsburgh, Pa.Continuation-impart of application Ser. No. 813,074, Apr. 3, 1969, nowabandoned.

[54] FORCE COMMUTATION CIRCUITS 14 Claims, 6 Drawing Figs.

[52] U.S.Cl 321/45 R, 32l/5,32l/45C [5l] Int. Cl HOZm 7/48 {50] FieldofSearch 321/5,45, 45 C [5 6] References Cited UNITED STATES PATENTS3,219,905 ll/l965 Davis et al. 321/45 C 3,399,336 8/1968 Koppelmann321/5 OTHER REFERENCES lBM Technical Disclosure Bulletin, Switch VoltageRegulator," Vol. 6, N04 8, January, 1964. p. 3 l- 32. (321/45 C) PrimaryExaminer-William M. Shoop, .lr. Attorneys- F. H. Henson, C. F. Renz andA. S. Oddi ABSTRACT: A commutation circuit is disclosed for commutatingpower-control switching devices, such as thyristors orsilicon-controlled rectifiers, utilized for supplying a load from a DCsource, wherein a commutating capacitor is operatively connected acrossthe'power control switching device to be commutated by selectivelyturning on commutatiomcontrolled switching devices, which may alsocomprise thyristors or silicon-controlled rectifiers. Alsounidirectional devices are employed to provide a circuit path for anyreactive energy trapped in the load circuit during the commutationoperation.

PATENTEUncT 19" mi SHEET 1 U? 3 FIG.3.

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John Rosa PAIENTEUnm 19 Ian FIG. 5.

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FIRING SEQUENCE VCA OUTPUT VOLTAG ES PATENTEUUCI man 3, 14,594

' SHEETBUF 3 FORCECOMMUTATION CIRCUITS CROSS-REFERENCE TORELATEDAPPLICATION This application is a continuation-in-part to applicationSer. No. 813,074, filed Apr.'3, 1969, and now abandoned.

BACKGROUND OF THE INVENTION 1. Field of thelnvention Thepresentinvention relates to commutation circuits and, more particularly,to commutation circuits of the force commutation type.

2. Discussion of the Prior Art A force-commutatedsystem, as compared toa naturally commutated system, is one in which the control-switchingdevices, such as thyristors or silicon-controlled rectifiers, are forcedto their nonconductive state by a commutation circuit separate fromthese devices. One type of force-commutating circuit for thyristorchoppers or inverters utilizes both capacitive and inductive componentswhich are used in combination to eflect the forced commutation. Theprincipal function of the capacitive components is to. store energywhich is later utilized to supply the load during commutation. Theinductive components are utilized to prevent the rapid discharge ofcapacitance. Another common technique to utilize the inductive andcapacitive components toform a resonant circuit so that by meansof aringing action these components are actively involved in the commutationoperation. The size and weight of the inductive componentmake suchdesignundesirable for airborne or underseas applications. Inatypicalinverter using forced commutation, one-half of the total weightmay be due to the commutation inductors.

Another type of forced commutation circuit does not utilize inductiveelements but requires a separate charging path for the commutationcapacitors used. The separate charging path includes a resistivecomponent which gives rise to substantial energy losses therein thatseverely limit the efficiency of the circuit.

SUMMARY OF THE INVENTION Broadly, the present invention provides acommutation circuit wherein the commutation of controlled-switchingdevices is effected without requiring inductive components adding weightto the circuit or resistive components dissipating energy.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of thechopper embodying the teachings of the present invention;

FIG. 2 is a waveform diagram used in explaining the operation of FIG. 1;

FIG. 3 is a schematic diagram of a chopper circuit employing anotherembodiment of the present invention;

FIG. 4 is a schematic diagram of an inverter utilizing the teachings ofthe present invention;

FIG. 5 is a waveform diagram used in explaining the operation of FIG. 4;and

FIG. 6 is a schematic diagram of another embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. I, a choppercircuit is shown wherein a load 2 is supplied from a battery B through apair of powercontrolled switching devices Spl and Sp2, which maycomprise thyristors, silicon-controlled rectifiers or other equivalentdevices. By turning on the power devices Spl and Sp2 a series circuit isprovided including the battery E and the load impedance 2, shown toinclude a resistor R and can inductor L. which may typically comprisethe inherent inductance of the load Z. The power devices Spl and Sp2should be selected to carry the desired load current i, at the voltagelevel employed within the circuit configuration.

Also referring to the waveform diagram of FIG. 2 assume that at a timeprior to the time :1 both the power devices Spl and Sp2 are conductive.Curve A of FIG. 2 shows the voltage across the power device Spl. In thatthe device Spl is in its conductive state at this time, only a smallvoltage drop, due to the forward junction drop thereof, will appear. Asubstantially similar voltage drop will appear across the power deviceSp2. The voltage across the load impedance Z will thus be substantiallythe voltage E of the battery as shown in curve B of FIG. 2. Curve C ofFIG. 2 shows the load current I), which is gradually increasingapproaching the time II. The capacitor C at a time prior to the time I1is charged as indicated on FIG. 1 and in curve D of FIG. 2 tosubstantially the battery voltage E.

The commutation circuit of FIG. 1 includes a pair ofcommutating-controlled switching devices Scl, Sc2 the commutatingcapacitor C and a pair of unidirectional devices DI and D2 shown asdiodes. The commutation control switching devices Scl and Sc2 maycomprise thyristors or silicon-controlled rectifiers having a powerrating depending upon the switching frequency employed for commutation.The anode of the commutation device Scl is connected to the positiveelectrode of battery E, and the cathode of the commutation device Sc2 isconnected to the negative electrode thereof. The cathode-anodeconnection of the device Sc I-Sc2 is connected to the left side of thecapacitor C. The right side of the capacitor C is connected to thecathode-anode connection of the diodes D1 and D2, with the cathode ofthe diode D1 being connected to the cathode of the power device Spl andthe anode of the diode D2 being connected to the anode of the powerdevice Sp2.

In order to commutate the power device Spl at the time 11, thecommutation device Scl is turned on thereby providing a conductivepathfrom the battery E, the anode-cathode circuit of the commutationdevice Scl, the capacitor C, the diode DI, the load impedance Z and thepower device Sp2. With the capacitor C charged is indicated in FIG. 1being connected in series with the load Z, the voltage across the load Zwill then jump to a value substantially equal to 2B as indicated incurve B of FIG. 2. The power-switching device Spl will thus have anegative voltage applied from the cathode to the anode thereof thusreverse biasing this device causing it to be commutated off. The reversebias of E across the power device Spl is shown in curve A ofFIG. 2.

During the time interval :1 and t2 the capacitor C charges to theopposite polarity, with the left side thereof now being positive,through the commutation device Scl, the diode DI, the load 2 and powerdevice Sp2. At the time :2, as shown in curve D of FIG. 2, the capacitorwill be charged to the E voltage as indicated. At this time, the loadcurrent i, freewheels through the diodes D2 and D1 with the voltageacross the load Z as shown in curve B of FIG. 2 being clamped to forwardvoltage drops of the diode D1 and D2. This permits the commutatingdevice Scl and the power device Sp] to recover and to be in theirtumed-off state and react for the next cycle of operation. The powerdevice Spl is reverse biased until the capacitor voltage C reaches zerobetween the times t1 and :2. If capacitor C is properly sized, this timeis of sufficient time length for the power device Spl to be turned off.The voltage across the power device Spl then increases to be forwardbiased to the battery voltage E at the time :2 and remains there untilthe time (3 when both power devices Spl and Sp2 are gated on. The timeperiod t2-t3 is appropriately chosen so as to obtain the desired averageload voltage.

To effect the commutation of the power device Sp2, at a time (4, thecommutation-controlled switching device Sc2 is turned on. The turning onof the commutation device Sc2 causes a reverse bias to be applied acrossthe power device Sp2. The capacitor C, with its left side positive, isconnected through the commutation device Sc2 in series with the load 2and thus discharged therethrough, with the capacitor C charging to thepolarity as shown in FIG. 1 via the load impedance Z and diode D2. Whenthe capacitor C has charged to the battery voltage E, the commutationdevice Sc2 will recover with the load current i, freewheeling throughthe diodes D2 and D1. The commutation circuit is thereby reset for thenext commutating cycle when the power device Spl will be commutated offby the turning on of the commutation device Scl as previously described.

The embodiment of FIG. 1 thus provides commutation of thepower-switching devices Spl and Sp2 without the requirement of anisolating or commutating inductor. The commutating capacitor C isconnected across the respective powerswitching device in order to turnit off while also being placed in series with load 2 so as to rechargeto the opposite polarity for operation during the next commutationcycle. The circuit of FIG. I also does not require a separate chargingresistor for the commutating capacitor C thereby providing a highlyefficient chopping circuit.

FIG. 3 shows another embodiment of the present invention employed in achopper circuit wherein only one power-controlled switching device Spois utilized. This single power device Spo is connected in series betweenthe battery E and the load Z so that a load current i will flow throughthe load impedance Z. The commutating circuit for the power device Spois shown to include four controlled-switching devices Sca, Sea, Scb andScb. The anodes of the devices Spo, Sea, and Scb are commonly connected,and the cathodes of the devices Spo, Scb and Sca' are commonlyconnected. The commutating capacitor C is connected between thecathode-anode junctions of devices Sca-Scb' and Scb-Sca'. A freewheelingdiode D is connected across the impedance 2 with the cathode thereofbeing connected to the cathode of the power device Spo.

Assume that the capacitor C is charged to the polarity as shown in FIG.3 to approximately the battery voltage E. With the power device Spoconductive, the load current i will be supplied therethrough from thebattery E. In order to commutate off the power device Spo, thecommutation devices Scb and Scb are turned on. This connects thecommutating capacitor C directly across the anode-cathode electrodes ofthe power device Spo with the cathode thereof being rendered positivewith respect to the anode due to the polarity of charge on the capacitorC. The power device Spo will thus be commutated off due to the reversebias. The capacitor C will discharge through the commutating devices Scband Scb and the load impedance Z and recharge from the battery E throughcommutating devices Scb and Scb to the opposite polarity so that thebottom side thereof will now be positive. When the capacitor C has fullycharged to substantially the battery voltage E, the load current I},will freewheel through the diode Do with the load voltage being reducedto substantially zero. Load voltage is reapplied by turning on the powerdevice Spo at the next cycle of operation. With the commutating circuitreset for the next commutation operation, commutation devices Sea andSca' are turned on which apply a reverse bias to the power device Spofrom the commutating capacitor C thereby turning off the power deviceSpo. The capacitor C recharges through the commutating devices Sea andSca' and the load Z to the polarity as shown in FIG. 3 with the devicesSea and Sea recovering when the capacitor C has charged to substantiallythe voltage E thereby resetting the commutating circuit. The currentfreewheels through the diode Do with the load voltage going to zero andthe chopper circuit being then ready for the next energizing cycle.

The circuit of FIG. 3 requires only one power switching device Spo whichis rated to carry full load current but requires four auxiliarycommutating devices. The circuit of FIG. I requires only four switchingdevices. Two of these must be rated for full power. The rating of thecommutation devices in each of the circuits depends upon the switchingfrequency. If a low-switching frequency is utilized, the circuit of FIG.3 should prove more economical since four relatively low-ratedcommutating devices could be utilized with the single full power deviceSpo. However, at higher switching frequencies, FIG. I should be moreeconomical since only two highfrequency commutating devices would berequired for operation at the high frequency.

FIG. 4 shows the commutation circuit of FIG. I being utilized with athree-phase inverter circuit. Thethree-phase inverter circuit isconnected in a standard bridge array and includes controlled-switchingdevices SAI, SA2, S81, S82, SCI and SCZ. The anodes of the bridgedevices SAI, S81 and SCI are connected to a 8+ bus which is at thecathode of line power device Spl. The cathodes of the bridge devicesSAZ, SBZ and 5C2 are connected to a 8- bus at the anode of the linepower device Sp2. A three-phase load is provided including the impedanceelements Za, 2b and Zc with one end of these elements being commonlyconnected and the other ends being connected to terminals Ta, Tb and Te,respectively. The terminals Ta, Tb and Tc are connected to thecathode-anode junctions of the controlled-switching devices SAl-SA2,881- S132, and SCI-8C2, respectively. A plurality of feedback controlswitching devices 5A2, 8A1, S32, S81, 8C2 and SCI are provided and arerespectively associated with the bridge switching devices 8A1, 8A2, SBI,SB2, SCI and SCZ. The anodes of the feedback devices SAl, S81 and SCIare commonly connected to the negative terminal of the battery E, whilethe cathodes of the feedback devices 8A2, S82 and SC2' are connected tothe positive electrode of the battery E. The cathode-anode junction ofthe feedback devices SAl-SA2bq.-, SBl'-SB2', SCI'SC2' are, respectively,connected to the terminals Ta, Tb and Te. The inverter circuit asdescribed is conventional except that feedback-switching devices 8A1,5A2, S81, S82, SCI and SC2' are utilized rather than conventionalfeedback diodes. By utilizing controlled switches, such assilicon-controlled rectifiers or thyristors, instead of diodes forhandling the reactive power and circulating current, a short circuitdischarge path for the commutating capacitor C through abridge-controlled switching device and the complementary feedback device(e.g., 8A1 and 8A2) is avoided. To accomplish this, bridgecontrolledswitches are turned on for the entire desired conduction period exceptfor the short time when the respective commutating-controlled switchdevice Scl or $02 is gated on. At these instants the complementaryfeedback control switch (having the same letter designation with aprime) is also fired with a short pulse. The result of this is that thefeedback device can take over conduction from a bridge device, but abridge device cannot go into conduction due to the firing of acommutating device Scl or Sc2 when the complementary device or itsassociated device is conducting which would otherwise cause a shortcircuit discharge of the commutating capacitor C.

Referring now to FIG. 5 which is a waveform diagram showing the firingsequence of the various controlled switches utilized in FIG. 4 and theoutput voltage developed across the terminals Ta, Tb and Tc, a specificexample of the mode of operation of FIG. 4 will be described.

At the time :1 in FIG. 5, the line power devices Spl and Sp2 areconductive along with the bridge devices SAI, S82 and SCI so that acurrent path is provided through these bridge devices to the impedanceelements Za, lb and Zc. At the time t2, it is necessary to commutate thebridge device SAI. In order to accomplish this, firing pulses areremoved from the line device Spl, the bridge device 8A1 and, for a smallinterval of time, from the bridge device SCI. Also at the time 12, thecommutating device Scl is gated on as are the complementary feedbackdevices SA! and SCI by short interval pulses as indicated by the firingsequence of FIG. 5. The line power device Spl is substantiallyinstantaneously turned off by the reverse bias supply thereacross fromthe commutating capacitor C which had previously been charged asindicated in FIG. 4. Current flow into the B+ bus is temporary throughthe commutating device Scl and the commutating capacitor C. It should benoted that if the gate signal had not been removed from the bridgedevice SCI at the time the capacitor C could instantaneously dischargethrough the devices SCI, 8G2 and Scl, which in certain-instances, wouldnot reverse bias the line device Spl for s sufficiently long time torecover. It therefore, in most instances, is desirable to take theprecaution of removing the gating pulse from the device SCI for a shortinterval of time. Also it should be; noted that the associated feedbackdevice SA2 to the bridge device SA2 is blocking during this time toprevent a short circuit for the capacitor C.

When the capacitor voltage has fully reversed to a polarity opposite tothat shown in FIG. 4, current into the terminal Ta transfer to thefeedback device SAl' which was gated on at the time 12. The bridgedevice SAl is thus commutated and the cornmutating circuit is reset forthe next commutating interval. The line device Spl is refired and thegating pulse reapplied to the bridge device 8C]. This then reestablishesthe operating conditions of the inverter until the period :3.

At the time B, the bridge device SA2 is gated on and is already to takeover conduction from the feedback device SAl when current through theimpedance element Za reverses.

At the time :4 the gate pulse is removed from the line device Sp2 andbridge devices 882 and'SA2. A short gating pulse is supplied to thecommutating device Sc2 to commutate the line device Sp2 thereby with thebridge device 882 also being commutated at this time. At the time :4,the feedback devices 882 and 8A2 are also fired with a short pulse inorder to permit flow of reactive power therethrough to the source E. Thecommutating capacitor C charges through the commutating device Sc2 andthe load to a polarity as shown in FIG. 4 with the commutating circuitbeing reset for the next commutating interval as previously described.

The commutating circuit as employed in FIG. 4 is operative to commutatethe line devices Spl and Sp2 as well as the various bridge devices 8A1,8A2, 8B1, 8B2, SCl and SC2 so that the three phase voltages VAB, VBC,and VCA are developed as shown in the output voltage portion of thewaveform diagram of FIG. 5. 2

FIG. 6 shows another embodiment of the present invention wherein thepower control switching devices Spl and Sp2 need only support one-halfof the source voltage B when being commutated off as opposed tosupporting the full voltage E as is in the embodiments of FIGS. 1, 3 and4. The advantage of this is that power-switching devices may be utilizedwhich have only approximately one-half the forward breakover rating ascompared to power devices which must sustain the full source voltage E.This is accomplished in the circuit of FIG. 5 by commutating both of thepower devices Spl and Sp2 at the same time rather than just one of thedevices as shown in the embodiments of FIG. 1 and FIG. 4.

The commutation of both power devices Spl and Sp2 at the same time inFIG. 6 is accomplished in the following manner. Assume that both powerdevices Spl and Sp2 are conductive with a load current i, being appliedthrough the impedance in the direction shown and that a pair ofcommutation capacitors Cl and C2 are charged to the indicated polaritiesto a voltage substantially equal to tE/Z as shown. To terminate thecurrent flow i through the-load Z, the power devices Spl and Sp2 arecommutated by turning on a pair of commutating-control switching devicesScal and Scbl by the application of gating pulses to the respective gateelectrodes thereof. Another pair of commutation control switchingdevices Sca2 and Scb2 are maintained in their nonconductive state. Thecommutation devices Scal and Sca2 are connected in series from anode tocathode between positive and negative terminals of the DC source E. Thedevices Sca2 and Scb2 are also connected from anode to cathode betweenthe positive and negative electrodes of the source E. With the turningon of the commutation device Scal, a reverse bias from the capacitor Clis applied across the cathode-anode circuit of the power device Splsubstantially equal to 5/2, with a diode Dal completing the connectionto the cathode of the device Spl. At the same time that the commutationdevice Sea] is turned on, the commutation device Scbl is also turned onwhich causes a reverse bias voltage the commutation capacitor C2 to beapplied across the power device Spl via the anode-cathode circuit of thecommutation device Scbl and the anode-cathode circuit of a diode Db 2,which is connected between the anode of the power device Sp2 and theright side of the capacitor C2. Thus by the firing of the commutationdevices Scal and Scbl both power devices Spl and Sp2 are commutated oil.

A current path is provided for the load current i being the commutationperiod from the position terminal of the source E, the commutationdevice Seal, the capacitor C1, the diode Dal, the load 2, a diode Db2,the capacitor C2, the commutation device Scbl to the negative terminalof the battery E. The polarity of charge across the capacitors Cl and C2reverses so that the right side of the capacitor C] will be negative andthe right side of thecapacitor of C2 will be positive, with themagnitude of the voltage thereacross being substantially equal to E/2.Thepower devices Spl and Sp2 are reversed biased until the time whenthecharging voltages for the capacitors Cl and C2 cross zero. When thecapacitors have charged to the magnitude E/2 of the reversed polarity,the load current i will freewheel through the pair of diodes Dal-Dbl andDa2 and Db2. The inductive energy in the inductance of the load Z willthus be dissipated and the voltage across the load Z will be equal tozero with both power devices Spl and Sp2 being in their blocked state.The commutation devices Seal and Scbl recover as the currenttherethrough goes to zero.

Both power devices Spl and Sp2 may now be refired in order to sustain adesired average voltage across the load 2, and the current will besupplied in a direction as shown on FIG. 5. When it is again desired tocommutate the power devices Spl and Sp2, the commutation-controlledswitching devices Sca2 and Scb2 are turned on while the devices Seal andScb2 remain in their turned oil condition. The commutation device Sca2connects the capacitor C2 across the power device Spl via the diode Da2to reverse bias the power device Spl and thereby turn it ofi'. The powerdevice Sp2 is commutated at the same time by the capacitor C1 beingconnected thereacross via the commutation device Scb2 and the diode Dbl.

Hence, with the commutation of the power devices Spl and Sp2, the loadcurrent i; passes through a path including the commutation device Sca2,the capacitor C2, the diode Da2. the load Z, the diode Db2, thecapacitor C1 and the device Scb2. The capacitors Cl and C2 will chargethe polarity as indicated on FIG. 5 to the magnitude E/2 with thereverse bias appearing across the power devices Spl and Sp2 until thecharge voltage crosses the zero axis. When the capacitors have chargedto the voltage 5/2, the load current I], will freewheel through thediode pairs Dal-Dbl and Da2-Db2 to dissipate the inductive energy in theinductor L of the load 2. The commutation devices Sa2 and Scb2 willrevert to their blocking condition with the circuit now being reset forthe next cycle of operation when the power devices Spl and Sp2 are gatedon.

It can thus be seen that through the use of an additional commutationcapacitor, two commutation-controlled switching devices and twoadditional diodes that both power devices Spl and Sp2 can be commutatedoff at the same time thereby only requiring each power device to sustainone-half of the source voltage E and thereby permitting the use of muchlower rated power devices.

lclaim:

l. In a circuit for supplying a load operative with a DC source having apositive output terminal and a negative output tenninal, the combinationcomprising:

i. first and second power-controlled switching devices each connectedbetween said load and a different one of said output terminals of the DCsource for supplying the load when both said power devices areconductive; and

II. a commutation circuit comprising:

a. a commutation capacitor chargeable to a first and a second polarity;first conduit means including a first commutation-con trolled switchingdevice and a first unidirectional device for selectively connecting saidcapacitor charged to said first polarity across the then conductivefirst power-controlled switching device when said firstcommutation-controlled switching device is turned on to commutate offthe first power device, said capacitor charging to said second polarityin response to the turning on of said first commutation controlledswitching device; and

c. second circuit means including a second commutationcontrolledswitching device and a second unidirectional device for selectivelyconnecting said capacitor means charged to said second polarity acrossthe then conductive second power-controlled switching device when saidsecond commutation-controlled switching device is turned on to commutatethe second power device, said capacitor charging to said first polarityin response to turning on of said second commutation-controlledswitching device.

2. The combination of claim 1 wherein;

a. each of said devices has a cathode and an anode;

b. a first conduit leg is connected across the source ends of said firstand second power controlled switching devices and includes said firstand second commutation-controlled switching devices connected in seriesand oriented in the same direction, the anodes of the first power deviceand the first commutation device being connected together, and thecathodes of the second power device and the second commutation devicebeing connected together;

c. a second circuit leg is connected across the load ends of said firstand second power devices and includes said first and secondunidirectional devices connected in series and oriented in the samedirection, the cathodes of the first power device and the firstunidirectional device being connected together, and the anodes of thesecond power device and the second unidirectional device being connectedtogether; and

d. one side of said capacitor is connected to a junction between saidfirst and second commutation-controlled switching devices, the otherside of the capacitor being connected to a junction between said firstand second unidirectional devices.

3. The combination of claim 2 which includes a DC-to-AC inverterconnected between said load and said power devices, said inverter havinga plurality of controlled-switching devices for operation in apredetermined pattern to connect DC to AC, said commutation circuitbeing operative for effecting selective commutation of thelast-mentioned switching devices.

4. The combination of claim 2 wherein said first and secondunidirectional devices are diodes.

5. The combination of claim 1 which includes a DC-to-AC inverterconnected between said load and said power devices, said inverter havinga plurality of controlled-switching devices for operation in apredetermined pattern to connect DC to AC said commutation circuit beingoperative for efiecting selective commutation of the last-mentionedswitching devices.

6. The combination of claim 1 wherein said first and secondunidirectional devices are diodes.

7. The combination of claim 1 wherein:

said first and second unidirectional devices are operatively connectedacross said load for pioviding a conductive path for the load currentwhen said first and second power devices are nonconductive.

8. The combination of claim 7 wherein said first and secondunidirectional devices are diodes.

9. The combination of claim 8 which includes a DC-to-AC inverterconnected between said load and said power devices, said inverter havinga plurality of controlled-switching devices for operation in apredetermined pattern to connect DC to AC, said commutation circuitbeing operative for effecting selective commutation of thelast-mentioned switching devices.

10. The combination of claim 1 which includes:

a three-phase inverter operatively connected between said first andsecond power devices and said load, said inverter includes,

a plurality of bridge-controlled switching devices for selectivelycompleting a bidirectional path to said load so that an alternating polhase out ut appears thereacross, a plurality of feedbac -control edswitching devices respectively associated with each of said bridgecontrolled switching devices and being in a blocking state associatedwith said plurality of bridge-controlled switching devices to becommutated off;

said commutation circuit operative for efiecting the selectivecommutation of said bridge-controlled switching devices.

11. The combination of claim 6 wherein:

each of said plurality of feedback devices being complementary to one ofsaid bridge devices other than the associated devices, saidcomplementary feedback devices being selectively turned on to provide areturn circuit path to said source when selected of said plurality ofbridge devices have been commutated.

12. In a circuit for supplying a load operative with a DC source havinga positive output terminal and a negative output terminal;

first and second power-controlled switching devices each connectedbetween said load and a different one of said output terminals of the DCsource for supplying the load when both said power devices areconductive;

first and second commutation capacitors respectively chargeableoppositely to first and second polarities with respect to one another;

first circuit means including a first commutation-controlled switchingdevice and a first unidirectional device for selectively connecting saidfirst capacitor when charged to said first polarity with respect to saidsecond capacitor across said first power device to commutate it off whensaid first commutation device is turned on;

second circuit means including a second commutation-controlled switchingdevice and a second unidirectional device for selectively connectingsaid second capacitor when charged to said second polarity with respectto said first capacitor across said second power device to commutate itoff when said second commutation device is turned on;

said first and second commutation devices being turned on atsubstantially the same time so that the voltage from said DC sourcedivides between said first and second power devices;

third circuit means including a third commutation-controlled switchingdevice and a third unidirectional device for selectively connecting saidfirst capacitor when charged to said second polarity with respect tosaid second capacitor across said second power device to commutate itoff when said third commutation device is turned on; and

fourth circuit means including a fourth commutationcontrolled switchingdevice and a fourth unidirectional device for selectively connectingsaid second capacitor when charged to said first polarity with respectto said first capacitor across said first power device to commutate itoff when said fourth commutation device is turned on;

said third and fourth commutation devices being turned on atsubstantially the same time so that the voltage of said DC sourcedivides between said first and second power devices when beingcommutated.

13. The combination of claim 12 wherein:

said first and third unidirectional devices and said second and fourthunidirectional devices are respectively connected across said load forproviding a conductive path for load current when said first and secondpower devices have been commutated.

14. The combination of claim 13 wherein said first, second, third andfourth unidirectional devices are diodes.

1. In a circuit for supplying a load operative with a DC source having apositive output terminal and a negative output terminal, the combinationcomprising: I. first and second power-controlled switching devices eachconnected between said load and a different one of said output terminalsof the DC source for supplying the load when both said power devices areconductive; and II. a commutation circuit comprising: a. a commutationcapacitor chargeable to a first and a second polarity; b. first conduitmeans including a first commutation-controlled switching device and afirst unidirectional device for selectively connecting said capacitorcharged to said first polarity across the then conductive firstpower-controlled switching device when said first commutation-controlledswitching device is turned on to commutate off the first power device,said capacitor charging to said second polarity in response to theturning on of said first commutation controlled switching device; and c.second circuit means including a second commutationcontrolled switchingdevice and a second unidirectional device for selectively connectingsaid capacitor means charged to said second polarity across the thenconductive second powercontrolled switching device when said secondcommutationcontrolled switching device is turned on to commutate off thesecond power device, said capacitor charging to said first polarity inresponse to turning on of said second commutationcontrolled switchingdevice.
 2. The combination of claim 1 wherein; a. each of said deviceshas a cathode and an anode; b. a first conduit leg is connected acrossthe source ends of said first and second power controlled switchingdevices and includes said first and second commutation-controlledswitching devices connected in series and oriented in the samedirection, the anodes of the first power device and the firstcommutation device being connected together, and the cathodes of thesecond power device and the second commutation device being connectedtogether; c. a second circuit leg is connected across the load ends ofsaid first and second power devices and includes said first and secondunidirectional devices connected in series and oriented in the samedirection, the cathodes of the first power device and the firstunidirectional device being connected together, and the anodes of thesecond power device and the second unidirectional device being connectedtogether; and d. one side of said capacitor is connected to a junctionbetween said first and second commutation-controlled switching devices,the other side of the capacitor being connected to a junction betweensaid first and second unidirectional devices.
 3. The combination ofclaim 2 which includes a DC-to-AC inverter connected between said loadand said power devices, said inverter having a plurality ofcontrolled-switching devices for operation in a predetermined pattern toconnect DC to AC, said commutation circuit being operative for effectingselective commutation of the last-mentioned switching devices.
 4. Thecombination of claim 2 wherein said first and second unidirectionaldevices are diodes.
 5. The combination of claim 1 which includes aDC-to-AC inverter connected between said load and said power devices,said inverter having a plurality of controlled-switching devices foroperation in a predetermined pattern to connect DC to AC saidcommutation circuit being operative for effecting selective commutationof the last-mentioned switching devices.
 6. The combination of claim 1wherein said first and second unidirectional devices are diodes.
 7. Thecombination of claim 1 wherein: said first and second unidirectionaldevices are operatively connected across said load for providing aconductive path for the load current when said first and second powerdevices are nonconductive.
 8. The combination of claim 7 wherein saidfirst and second unidirectional devices are diodes.
 9. The combinationof claim 8 which includes a DC-to-AC inverter connected between saidload and said power devices, said inverter having a plurality ofcontrolled-switching devices for operation in a predetermined pattern toconnect DC to AC, said commutation circuit being operative for effectingselective commutation of the last-mentioned switching devices.
 10. Thecombination of claim 1 which includes: a three-phase inverteroperatively connected between said first and second power devices andsaid load, said inverter includes, a plurality of bridge-controlledswitching devices for selectively completing a bidirectional path tosaid load so that an alternating polyphase output appears thereacross, aplurality of feedback-controlled switching devices respectivelyassociated with each of said bridge controlled switching devices andbeing in a blocking state associated with said plurality ofbridge-controlled switching devices to be commutated off; saidcommutation circuit operative for effecting the selective commutation ofsaid bridge-controlled switching devices.
 11. The combination of claim 6wherein: each of said plurality of feedback devices being complementaryto one of said bridge devices other than the associated devices, saidcomplementary feedback devices being selectively turned on to provide areturn circuit path to said source when selected of said plurality ofbridge devices have been commutated.
 12. In a circuit for supplying aload operative with a DC source having a positive output terminal and anegative output terminal; firsT and second power-controlled switchingdevices each connected between said load and a different one of saidoutput terminals of the DC source for supplying the load when both saidpower devices are conductive; first and second commutation capacitorsrespectively chargeable oppositely to first and second polarities withrespect to one another; first circuit means including a firstcommutation-controlled switching device and a first unidirectionaldevice for selectively connecting said first capacitor when charged tosaid first polarity with respect to said second capacitor across saidfirst power device to commutate it off when said first commutationdevice is turned on; second circuit means including a secondcommutation-controlled switching device and a second unidirectionaldevice for selectively connecting said second capacitor when charged tosaid second polarity with respect to said first capacitor across saidsecond power device to commutate it off when said second commutationdevice is turned on; said first and second commutation devices beingturned on at substantially the same time so that the voltage from saidDC source divides between said first and second power devices; thirdcircuit means including a third commutation-controlled switching deviceand a third unidirectional device for selectively connecting said firstcapacitor when charged to said second polarity with respect to saidsecond capacitor across said second power device to commutate it offwhen said third commutation device is turned on; and fourth circuitmeans including a fourth commutation-controlled switching device and afourth unidirectional device for selectively connecting said secondcapacitor when charged to said first polarity with respect to said firstcapacitor across said first power device to commutate it off when saidfourth commutation device is turned on; said third and fourthcommutation devices being turned on at substantially the same time sothat the voltage of said DC source divides between said first and secondpower devices when being commutated.
 13. The combination of claim 12wherein: said first and third unidirectional devices and said second andfourth unidirectional devices are respectively connected across saidload for providing a conductive path for load current when said firstand second power devices have been commutated.
 14. The combination ofclaim 13 wherein said first, second, third and fourth unidirectionaldevices are diodes.